The invention relates to a fibrous reinforcement for use in fiber composites wherein crossover points of fibers are bonded together using threads or other elements. The invention relates also to a process for producing fiber composites comprising a matrix material and a fibrous reinforcement embedded in the matrix material, wherein the fibers in the fibrous reinforcement are bonded together using threads or other elements.
Fiber composites are materials comprising a matrix material, for example, a polymer matrix as a continuous phase, and embedded reinforcing fibers as a discontinuous phase. The reinforcing fibers bound into the matrix material improve the properties of the matrix material, i.e., of the plastic. More particularly, the strength and stiffness of the material can be advantageously modified.
The fibrous reinforcement used is typically in the form of a sheetlike arrangement of fibers. Sheetlike arrangements of fibers are also known as textile sheet materials or fabrics and can be subdivided into three groups:    a) webs;    b) noninterlooping systems (wovens, scrims, braids);    c) interlooping systems (loop-formed knits, loop-drawn knits).
Depending on the method used to construct or produce these sheet materials, the fixation of the actual reinforcing fibers takes different forms. Owing to their structure, for example, loop-drawn knits, loop-formed knits and wovens normally require no additional fixation. Webs, in contrast, may be needled or fixed (bonded) by means of a binder. Nonwoven scrims, which are formed by superposition of differently oriented fiber or filament systems, require fixation of the crossover points of individual fibers to be handleable.
After impregnation and curing or solidification of the matrix material, the “thread” (which, as used herein, can include other fixation elements) is still present in fiber form in the composite as an additional phase and accordingly possesses an interface with the matrix material.
It has been determined that the thread may be an imperfection and a possible initiation point for damage, owing to the different physical properties of the thread material, compared with the matrix material and compared with the actual reinforcing material, and owing to possibly inadequate attachment of the thread to the matrix material. If, as a result of the thread having an interfacial area, the surface of the stitching thread becomes detached from the matrix material, it is likely that, similarly to other pores, dynamic stress on the fiber composite will lead to areas of damage emanating from the thread and possibly to complete failure of the fiber assembly.
Furthermore, matrix materials where the infiltration or curing requires that they be exposed to an elevated temperature above room temperature or above the storage temperature of the fibrous reinforcement are problematical in that, beyond a certain temperature, the shrinkage tendencies of the thread will be responsible for distortions or distensions being introduced into the actual reinforcing fibers as a function of the mechanical and/or thermal processing history of the thread. This risk of the fibrous reinforcement being distorted is always present when the thread undergoes shrinkage prior to the actual gelling or solidifying of the matrix material.